1. Field of the Invention
This invention is generally directed to the production of monopotassium phosphate from commercial wet process phosphoric acid and more specifically to a method of producing a relatively pure low chloride content monopotassium phosphate by a process wherein the wet process phosphoric acid is mixed with potassium chloride and heated to form a melt from which metallic impurities may be recovered by settling, filtration, centrifugation and other by-product recovery techniques. In some instances where double salts of phosphorous are present in the recovered by-products, such by-products are recycled back to the acid melt The filtrate remaining after the impurities are removed is further processed to separate the monopotassium phosphate from the mother liquor. The separation of the monopotassium phosphate may be accomplished in several manners including evaporative or cooling crystallization in a single or plural stages wherein the crystalline monopotassium phosphate is separated in the first stage and, if desired, potassium sulfate and other by-products recovered in subsequent stages. The wash liquors from the crystalline steps may be returned to the monopotassium phosphate melt.
Utilizing the process of the present invention, relatively pure monopotassium phosphate may be obtained having chloride levels as low as 0.1%.
2. History of the Related Art
There have been many processes proposed to produce monopotassium phosphate by a direct reaction of commercial wet process phosphoric acid and potassium chloride An important factor in most processes for producing monopotassium phosphate is to obtain a product having a low content of chloride However, many problems have been encountered when using commercial wet process phosphoric acid as an initial acid source.
In most prior art processes, the desired degree of chloride reduction was accomplished by driving chlorides off as gases to form hydrochloric acid (HCL) with the initial melt of wet process phosphoric acid and potassium chloride being maintained at high temperatures for long periods of time. Such processes have hertofore not been completely successful or satisfactory due to the creation of insoluble polyphosphates. To avoid this loss of phosphates, some processes have attempted to achieve the desired reduction in chloride levels at reduced temperatures and retention times. Other processes have sought to prevent the formation of precipitates from the impurities which are in the feed acid.
In the wet process formation of phosphoric acid, phosphate rock is reacted with phosphoric acid and sulfuric acid. As there are many impurities in the rock, such impurities are also contained in the wet process phosphoric acid. These impurities not only include various organic materials but various metal and other elements including iron, aluminum, magnesium, potassium, cadmium, fluorine, calcium and sodium. These impurities present unique problems in the processing of wet process phosphoric acid with potassium chloride in order to obtain relatively pure monopotassium phosphate products.
In U.S. Pat. No. 3,554,729 to Curless, a process for producing low chloride potassium phosphate fertilizers from wet process phosphoric acids is disclosed wherein the phosphoric acids are mixed with potassium chloride and minor amounts of sulfuric acid. The mixture is heated between 70.degree. C. and 200.degree. C. and the chlorides are recovered as hydrochloric acid from the melt by passing a stream of non-reactive gas or steam therethrough while maintaining the melt in an atmosphere of reduced pressure. In this manner, a resultant potassium acid phosphate is obtained which may be formulated into concentrated fertilizers having not more than 2% chloride. Although the process utilizes low temperatures, the method is directed to substantially eliminating chlorides by gas sparging at reduced pressures. Further, there is no attempt to create a relatively pure monopotassium phosphate by removing the impurities from the melt.
In U.S. Pat. No. 1,456,850 to Hazen et al., a process for preparing a monoammonium and monopotassium phosphate fertilizer in which the ratio of nitrogen (N), phosphoric acid (P.sub.2 O.sub.5) and potash (K.sub.2 O) are approximately 2-10-4 is presented. In this process, concentrated phosphoric acid (70 to 90 percent) is mixed with potassium chloride and heated to approximately 200.degree. C. Hot air is blown through the melt to accelerate the removal of chlorides from solution. Thereafter, a gaseous ammonia is added to neutralize the acids in the melt. The material in solution is centrifuged so as to remove precipitates of phosphates of potassium and ammonium. Due to the high concentration of the phosphoric acid, there are fewer impurities in the melt. Also, there is no attempt to deal with potential impurities as the ammonia is added to neutralize acids and recover the phosphate and potassium directly from the melt. A similar process for treating a melt of concentrated phosphoric acid and potassium chloride is disclosed by the same inventors in U.S. Pat. No. 1,456,831.
The use of an ammonia to neutralize acids in the production of fertilizers from wet process phosphoric acid mixed with potassium chloride is widely recognized. Other patents involving this general technology include U.S. Pat. No. 3,388,966 to MacDonald wherein monoammonium phosphate and diammonium phosphate are produced and U.S. Pat. No. 3,726,660 to Drechsel et al. wherein nitric acid is used to process phosphate rock to form a mixture which is reacted with potassium chloride and sulfuric acid. Upon removal of calcium sulfate precipitate, the resulting filtrate is contacted with ammonia to form a concentrated aqueous nitrophosphate fertilizer. Also, in U.S. Pat. No. 4,158,558 to Thompson et al., wet process phosphoric acid and potassium chloride are mixed, heated and sparged with steam in a multi-step process to recover hydrochloric acid and thereafter ammonia is added to maintain a desired pH level. The product stream is subsequently cooled and filtered to obtain a potassium polyphosphate liquid fertilizer.
In U.S. Pat. No. 3,547,615 to Beckham, potassium phosphate fertilizer solutions are produced by reacting potassium chloride with 80% furnace grade phosphoric acid in the presence of steam in order to recover hydrochloric acid. The effluent is diluted, and thereafter introduced into a crystallizer-cooler to precipitate monopotassium phosphate. The monpotassium phosphate is subsequently treated with ammonia to form several liquid phases which are separated to obtain a heavy liquid phase tri-potassium phosphate product stream. Such a process, however, is not functional if the initial reactants were to include commercial grade wet process phosphoric acid.
Other prior art patents of interest include U.S. Pat. Nos. 3,767,770 to Worthington, 3,607,213 to Jerzy et al., 3,784,367 to Moore and Re. 29,450 to Drechsel et al.
The process which is the subject of the present invention allows the production of a low chloride product under milder conditions in the reaction system and provides for removal of the impurities introduced with the phosphoric acid, which allows a high quality crystal form of monopotassium phosphate to be recovered. It was discovered that the impurities in the melt was the reason crystallization of the monopotassium phosphate could not be achieved in prior processes utilizing wet process phosphoric acids. Prior treatment of the wet process acid for removal of organics is also not required, unless recovery of a high quality hydrochloric acid by-product is desirable. Even then, prior treatment of the acid may not be required, since the hydrochloric acid can be recovered in stages. Additionally, it is possible to treat the hydrochloric acid solution directly to remove the organics.